Here's the schematic - very simple. Outputs are darlingtons, the bias diodes are included in the package. Rbias could be 200 ohm trimpot, typically ends up in 10-30 ohm range. The outputs are mounted on the back of the board against the heatsink. The 330pf caps probably compensate for the airwiring. And I know the circuit is a little over-protected.
Prototype 1 used the SAP15 transistors which include the emitter resistors - but the SAPs are out of production. Prototype 2 used parallel pairs of STDs, but stability, or rather docility, was an issue - probably due to wiring.
Funny, it's been 40 years since I've done any power amp work (other than repairs) and very little has changed.
Even funnier, the pad that I used to jot down the rough schematic is the same one I used in college - 40 years ago !
I've included a schematic of the KR-9600 output board, but it's a bit hazy. Anyway, pins 1 & 10 are power, pins 3 & 8 are drive, 5 is output, 6 is ground.

STD03N and STD03P you used - and you have the diode and cap circled along with the transistor. Is that cos its included in the transistor already ?

And why is the circuit board on the heat sink and not the transistor ? Is that just for fit ?

I've seen these devices used before. Two output devices leave precious little room for oddball phase angles. I'd not consider this a very 'robust' modification, though it will bring the unit back to life (at least until a difficult speaker is connected, or someone gets too frisky with the volume control).